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An MeV gamma-ray imaging detector which makes a reconstruction of single photons possible has been developed. This MeV gamma-ray detector is a hybrid detector of a micro time projection chamber (micro-TPC) with a gaseous micro pixel chamber (μ-PIC) readout and an enclosing scintillation camera. We developed a prototype of the MeV gamma-ray imaging detector with the micro-TPC of 10 × 10 × 8 cm2 volume and an NaI(TI) scintillator of 10 × 10 × 2.5 cm3 size read by 25 single anode PMTs of 3/4' diameters, and succeeded in reconstructing the gamma-ray events with a radioactive source. The angular resolutions of the prototype detector were limited mainly by the tracking accuracy of the micro-TPC for minimum ionizing particles (MIPs) which is determined by the gas gain of the μ-PIC. We have, thus, developed an electrode structures of the μ-PIC using a new manufacture technology in order to improve the gas gain. As a result, a gas gain which is three-times higher than that of the previous μ-PIC has been obtained. This result is in good agreement with the simulation. Owing to this improvement of the electrode structures, not only the high gas gain but also a good gain uniformity has been achieved. [ABSTRACT FROM AUTHOR]

The energy spectrum of cosmic-ray antiprotons (¯p's) from 0.17 to 3.5 GeV has been measured using 7886 ¯p's detected by BESS-Polar II during a long-duration flight over Antarctica near solar minimum in December 2007 and January 2008. This shows good consistency with secondary ¯p calculations. Cosmologically primary ¯p's have been investigated by comparing measured and calculated p spectra. BESS-Polar II data show no evidence of primary ¯p's from the evaporation of primordial black holes. [ABSTRACT FROM AUTHOR]